This invention relates to apparatus used in the maintenance of fiber optic communication networks.
Such networks include a multiplicity of stations, called repeater huts, into each of which a plurality of fiber optic cables extend. Each such cable, which extends underground, contains a multiplicity of fiber optic strands, which carry the communication signals. Stations, or repeater huts, are underground chambers located perhaps 40 to 50 kilometers apart. The signals coming into a station on one cable are amplified, switched, and otherwise regenerated inside each hut, and then sent out on another cable.
Optical Time Domain Reflectometers (OTDRs) are, in effect, "laser radar" instruments which test the condition of fiber optic strands by injecting laser beam pulses into the strand at one point, and then receiving their returned reflections. The time elapsed from the sending to the returning of the pulse indicates the distance from the OTDR to the point of reflection. Variations in the energy of the returning pulses are used to diagnose the condition of the fiber optic strand. These diagnoses are used for several purposes: accurate splice analysis, pinpointing fault locations, cable acceptance testing, cable installation, and end-to-end tests. Most of these purposes relate to installation and testing of the initial system.
However, the pinpointing of fault locations is of extreme importance when damage accidentally occurs to an existing cable installation, usually as the result of mechanical accident.
OTDRs may be carried to the sites of work, for use in diagnosing fiber optics cable problems. However, the assignee of this application has pioneered a remote control OTDR system, in which a central computer is separately linked to a plurality of OTDRs, each of which is located inside a station, or repeater hut, and each of which has its own computer linked to the central computer. Such a system provides many advantages, such as continuous remote monitoring of the cable links, triggering of an alarm if a problem occurs, and fast fault location from a remote central office terminal.
One problem with the remote control OTDR system is its expensiveness, due to the need for a separate OTDR at each station in the network.